Clinical nerve conduction studies have been used for many years as a diagnostic technique in the field of neurology. As a part of these diagnostic techniques, electromyograms (EMGs) are produced in response to nerve stimulation by electrical means. By use of EMGs, nerve condition velocities can be measured and analyzed as an aid in the diagnosis and treatment of various neurological disorders. For example, the blink reflex can be recorded using surface electrodes placed on the orbicularis oculi muscle after stimulation of the trigeminal nerve at the supraorbital foramen. In measuring the blink reflex using electrophysiological techniques, the active recording electrode is generally located approximately two centimeters away from the cathode of the stimulating electrodes. Another example of the use of electrophysiological techniques in measuring nerve conduction is the measurement of sensory, nerve and muscle action by stimulation of the median nerve at multiple points along its course between the palm and distal portion of the forearm. Generally, this technique involves measurements in increments of one centimeter, and the minimal distance between the stimulating and recording electrode is usually two to three centimeters. With this minimal distance, the time lapse from stimulus to response (latency) is usually less than two milliseconds.
The foregoing techniques are described in detail in various technical papers, especially those involving the work of Dr. Jun Kimura. Some of Dr. Kimura's work is described in the Journal of the Neurological Sciences, 1978, 38:1-10; in the Archives of Neurology, April 1977, 34:246-249; in the Electroencephalography and Clinical Neurophysiology, 1978, 45:789-792. The latter publication was co-authored by me and describes some aspects of my invention as set forth hereinafter.
Most electrode amplifiers used in electrophysiology recover from an overloading input to near quiescent baseline in five to ten milliseconds or more depending on the amplifier design and the amount of overload. Thus, in situations such as those referred to in the foregoing examples where the stimulus is coincident with an electrical event of sufficient magnitude to cause an overloading artifact, it is not possible to accurately record responses of latency less than several milliseconds. Thus, the accuracy and utility of EMGs using prior art electrode amplifiers is somewhat limited.
To a certain degree, the deficiencies of prior art electrode amplifiers can be partially avoided by using mechanical stimulation or by attempting to improve stimulus isolation. Also, modification of an AC amplifier and use of a compensator to offset the artifact can also be used to improve the accuracy of the recorded responses. Most electrode amplifiers consist of several stages in order to achieve the required gain, and the stages are AC-coupled to block DC operating levels. Usually, one or more of the coupling capacitors is variable to adjust the low frequency cutoff. In the quiescent state, there is a charge on each of the coupling capacitors according to the DC voltage difference between the stages. If an overloading input saturates the first stage, the charge on the coupling capacitor to stage two will change. When the overload is gone and the first stage returns to normal, this voltage difference, amplified by the succeeding stages, results in a large or sometimes saturated deflection at the amplifier output. This offset than decays at a rate set by the linear coupling time-constant and thus accounts for a long recovery time. Attempts have been made to reduce the recovery time of an amplifier by shortening the coupling time constants and/or reducing the voltage gain. However, this results in a loss of low frequency response or a loss in sensitivity or both. Thus, attempts to reduce the recovery time of prior art amplifiers to the point where they meet all the specifications required for electrophysiological recording have not met with success.
There is therefore a need for a relatively simple and inexpensive amplifier which recovers from overload in less then one millisecond without user adjustment, while still meeting the specifications required in clinical electrophysiology.